1. Field of the Invention
The present invention relates to an image forming apparatus, an image forming method, a computer program, and a recording medium. For example, the present invention relates to an image processing operation for adjusting the color tone of a monochrome image.
2. Description of the Related Art
An inkjet recording apparatus that uses a plurality of inks having different colors is known as an example of a recording apparatus capable of outputting a color image. The inkjet recording apparatus forms an image by a subtractive color mixing method and generally uses three basic colors consisting of cyan (hereinafter abbreviated as ‘C’), magenta (hereinafter abbreviated as ‘M’), and yellow (hereinafter abbreviated as ‘Y’). Use of this combination of colors allows representation of not only hues of C, M, and Y but also various other hues. For example, red (hereinafter abbreviated as ‘R’) can be represented by mixing M and Y. By gradually changing the proportions of the inks mixed together, colors over an entire color space can be substantially represented.
Recently, inkjet recording apparatuses have not only achieved a color image quality equivalent to that of film photography but also provided applications in the field of monochrome photography.
In the field of monochrome photography, the required color accuracy is extremely higher than that in the field of color photography. This is because human vision is sensitive to achromatic colors, and therefore even a slight color change caused by the brightness of a printed image (hereinafter called “color transition”) will be noticeable to a viewer. In order to solve this problem, a method for preventing the color transition has been suggested. According to this method, achromatic colors, such as black (hereinafter abbreviated as ‘K’) and gray (hereinafter abbreviated as ‘Gr’), are used more dominantly than inks of chromatic colors (see, for example, Japanese Patent Laid-Open No. 2005-238835).
Japanese Patent Laid-Open No. 2006-86708, for example, discusses another method for solving the above-described problem. According to this method, a maximum usage of chromatic inks is set in advance and an amount of ink ejected onto a recording medium relative to a change in a luminance signal is reduced, so that the color adjusting accuracy can be increased.
On the other hand, recently, grayscale printing has been increasingly attracting the users' attention. Accordingly, recording apparatuses capable of performing grayscale printing with different color tones have been suggested. Here, grayscale printing is equivalent to monochrome-mode printing. In grayscale printing, amounts of color inks used together with achromatic inks, such as black ink, are suitably adjusted so that a color tone is slightly changed from that of a normal grayscale (hereinafter called a “neutral tone”). For example, a gray color tone having warm colors, such as red, orange, and yellow, mixed therein (hereinafter called a “warm tone”) and a gray color tone having cold, inorganic colors, such as blue, mixed therein (hereinafter called “cool tone”) can be provided. The addition of small amounts of chromatic inks for tone adjustment to the achromatic inks that are dominantly used provides a color reproduction range that covers the warm tone and the cool tone.
Japanese Patent Laid-Open No. 2004-142423 also discusses a technique for performing monochrome-mode printing with different color tones. According to this technique, with reference to the combination of inks and amounts of recording thereof for obtaining the neutral tone, the amounts of recording of the chromatic inks are changed while the amounts of recording of the achromatic inks are maintained constant.
Recently, the required image quality of inkjet printers has been continuously increased. In particular, in monochrome mode, there have been demands for eliminating the color tone difference caused by “color transition”. In addition, with respect to density variation, a gradation performance higher than that in the color mode has been demanded. In the technique according to Japanese Patent Laid-Open No. 2004-142423, the amounts of recording of the chromatic inks are changed while the amounts of recording of the achromatic inks are maintained constant. Therefore, if an image corresponding to a certain luminance signal is printed in a plurality of monochrome modes corresponding to different color tones, such as the warm tone and the cool tone, the printed images have different densities depending on the monochrome modes even when the images are printed on the same kind of recording media.
As an example, a case will be considered in which an image is printed in a plurality of monochrome modes corresponding to the neutral tone, the cool tone, and the warm tone on the same kind of recording media. In this specification, a CIE-L*a*b* color space, which is commonly known, is used as the scale of color.
First, a process of representing the cool tone on a certain recording medium (hereinafter called a “recording medium A”) will be described below. In this process, according to the technique of Japanese Patent Laid-Open No. 2004-142423, the cool tone is represented by changing the amounts of recording of the chromatic inks of C, M, and Y relative to those in the neutral tone without changing the amounts of recording of the achromatic inks. In more detail, the color is changed by increasing the amount of recording of the C ink. As a result, the total amount of recording of the chromatic inks is increased from that in the neutral tone. Due to the above-described process, the value of b* in the CIE-L*a*b* color space of the cool tone becomes smaller than the value of b* in the CIE-L*a*b* color space of the neutral tone.
However, in the above-described method for representing the cool tone, the density is changed from that of the image in the neutral tone by the amount corresponding to the increase in the amount of C ink. Thus, the density of the image recorded on the recording medium differs from that of the image in the neutral tone.
Next, a process of representing the warm tone on the recording medium A will be described below. Also in this process, according to the technique of Japanese Patent Laid-Open No. 2004-142423, the warm tone is represented by changing the amounts of recording of the chromatic inks of C, M, and Y relative to those in the neutral tone without changing the amounts of recording of the achromatic inks. In more detail, the color is changed by increasing the amount of recording of the Y ink and reducing the amount of recording of the C ink. Due to the above-described process, the value of b* in the CIE-L*a*b* color space of the warm tone becomes larger than the value of b* in the CIE-L*a*b* color space of the neutral tone.
However, in the above-described method for representing the warm tone, the proportions of the chromatic inks are changed from those in the neutral tone, and the density of the image formed on the recording medium differs from that of the image in neutral tone. Thus, even when the images are formed on the same recording medium, the density differs between the neutral tone and the warm tone and between the warm tone and the cool tone. This can cause discomfort to the user.
On the other hand, when an image is printed on a recording medium A and a recording medium B having different colors (paper whites) in a certain monochrome mode, the recording density differs between the images recorded on the recording media A and B.
A case in which this problem becomes more significant will be described below.
Recently, inkjet printers have become capable of printing on various types of recording media to satisfy user's needs. The various types of recording media have different printing characteristics. The most important characteristic in monochrome-mode printing is the white of the recording medium itself, that is, the color represented when no ink is ejected onto the recording medium (hereinafter called “paper white”).
The reason why the paper white is important will be described below.
In inkjet printers, ink adheres to the recording medium in units of dots. Therefore, the grayscale is expressed by the density of the dots. A high-density area is obtained by recording a large number of dots per unit area, and a low-density area is obtained by recording a small number of dots per unit area, as is commonly known. Therefore, the color of the low-density region, in which the number of dots per unit area is relatively small, is largely influenced by the paper white, that is, the color of the regions where no dots are recorded.
An example is considered in which the paper white of the recording medium A is more yellow than that of another recording medium (hereinafter referred to as a “recording medium B”). The state in which “the recording medium A is more yellow than the recording medium B” is equivalent to the state in which the value of b* of the paper white of the recording medium A in the CIE-L*a*b* color space is more than the value of b* of the paper white of the recording medium A in the CIE-L*a*b* color space.
As described above, the influence of the paper white is large in the low-density area where the number of dots is small. Therefore, when the same amount of ink is ejected onto the recording medium A and the recording medium B, the recording medium A appears more yellow in the low-density areas. The appearance of the recording medium A can be made closer to that of the recording medium B by increasing the amounts of recording of C and M in the low-density areas of the recording medium A compared to those of the recording medium B. In this manner, the difference in the paper white can be compensated for.
The case in which the cool tone is represented on the recording medium A after the neutral tone is represented in the above-described manner will be described below. In this case, the amounts of recording of the chromatic inks (C and M in this case) must be further increased. As a result, the recording density is changed from that of the neutral tone, and the above-described problem that the density differs between the neutral tone and the cool tone and between the warm tone and the cool tone due to the variation in the amounts of recording of the chromatic inks becomes more significant. For example, a tone difference that can be represented in the warm tone cannot be represented in the cool tone. This causes discomfort to the viewer.
In addition, when a cool tone image is formed on two different kinds of recording media, the density of the cool tone image on one of the recording media can be different from that of the cool tone image on the other recording medium. As a result, there is a risk that a tone difference that can be represented on the recording medium B cannot be represented on the recording medium A, which also causes discomfort to the viewer.
With respect to the above-described problems, Japanese Patent Laid-Open No. 2004-142423 does not provide measures for adjusting the density difference between images formed on the same kind of recording media in different monochrome modes or between images formed on recording media with different paper whites in the same monochrome mode.